This invention relates to methods and apparatus used for accurately measuring the temperature of remote objects by a non-contact method, and more particularly, to such a method and apparatus which can determine and correct for all critical variables, especially for surface imperfections, of such objects under investigation and their environs.
There is a continuing need for improvements in radiation thermometry or in devices which can quickly, accurately and continuously measure the true or thermodynamic temperature of remote objects, either incandescent or at ambient temperatures, of various types without undue complications. Many manufacturing industries could improve efficiency, product or production quality and consistency, and save energy by more precise and accurate control of temperature at various stages in the process. While pyrometers and other radiation measuring devices have sometimes been employed, there are several factors which limit the usefulness of these devices.
It is well known that the measurement of a thermodynamic temperature by non-contact radiation thermometry requires knowledge of the spectral emissivity of the object. Emissivity, which is a function of both temperature and wavelength, is traditionally measured by comparing the emittance of a radiating body to that of a black-body at a given wavelength and temperature. In addition, however, emissivity is dependent upon the optical or surface characteristics of the object, which may vary from one location on the object to another, or which may vary over a period of time. Factors which can affect the spectral emissivity include the degree of surface roughness, the chemical nature of the surface, and the environment, finally, its intrinsic optical properties. These factors cannot be estimated reliably, calculated from known principles, or compensated for by traditional or known methods. Also, background radiation, i.e., radiation from sources other than the object being evaluated, may result in measurement errors of brightness, since the radiation may enter the detector, and the object may appear brighter than it actually is.
In view of the above, the measurement of the thermodynamic temperature of radiating surfaces by noncontact radiation thermometry must incorporate spectral emissivity data. Any method which does not incorporate such data together with radiance brightness measurement is prone to large errors, particularly when the surface emissivity is not known or changes with time or temperature.
In summary, major shortcomings in this field in the past have been the inability to compensate for variations of reflectivity and/or emissivity of the object and variations in background radiation.